Electromagnetic transducers have been used in magnetostatic structures in a wide range of applications, such as loudspeakers, microphones, generators, electric guitar pick-ups and the like. The generation of a voltage in a conductor by the changing of the magnetic flux of a magnetostatic structure or the movement of magnetic flux of a magnetostatic structure relative to the conductor is an old concept. These devices in the prior art commonly use a permanent magnet made of electrically conducting metal. Since magnets made of electrically conducting metals rapidly attenuate any electromagnetic energy, as do any electrical conductors, the use of such metal permanent magnets has been primarily limited to core elements inside electrically insulated conductor coils or applications as permanent magnets surrounding a floating speaker coil or similar applications.
With the advent of ceramic magnetic materials, magnets which are not electrical conductors have become available. Ceramic magnets are available in both permanent (hard) magnetic materials or magnetically soft materials of high permeability. Various types of ceramic magnetic compositions of both the hard and soft types use "ferrite" materials. Generally, these materials are magnetically soft materials (that is, non-permanent magnetics.) "Hard" or permanent magnetic materials are high loss, high retentivity, high coercivity materials with low permeability.
The coercive force of hard magnetic materials is on the order of tens of thousands of times greater than that of the lowest coercive force of soft magnetic materials. From a magnetic softness viewpoint, the important thing to regard is the hysteresis loop. For soft magnetic materials, the area of the hysteresis loop is quite small, whereas for "hard" magnetic materials, the area of the hysteresis loop is large by comparison with soft magnetic materials. The bulk of the work in electric circuit design using magnetic materials involves the application of magnetically soft cores in inductors and transformers and the like. These uses encompass a large range of ferrite and metal cores, and the applications of permanent magnets (metal or ceramic) in electronic circuit design has been nearly neglected.
In the Eugene A. Albright U.S. Pat. No. 4,429,314, the use of permanently magnetized, magnetically hard dielectric materials to obtain significantly improved operating characteristics in transmitting and receiving antennae is disclosed. In this patent, elongated electrical conductors are embedded in or encased in permanently magnetized magnetically hard dielectric materials to form antennae. The magnets include various ratios of ferrite powder, such as barium ferrite or cobalt ferrite in a suitable epoxy or thermal setting resin. The ferrites are suspended in the resin and then are permanently magnetized. The ratio of the barium or cobalt powder to the resin is disclosed as varying from a ratio of approximately 20% of the ferrite powder by weight to 90%. As disclosed in this patent, the preferred ferrites are isotropic or anisotropic barium ferrites, such as Ferroxdure and the like.
Even though "hard" magnetic materials are known, such materials have not been used in electromagnetic transducers such as microphones, guitar pick-ups, speakers and the like. Such devices generally employ "soft" magnetic material or permanent magnets made of electrically conducting metal. As stated previously, the conductivity of such metal permanent magnetics rapidly attenuates electromagnetic energy; so that metal permanent magnets cannot be used in many electromagnetic transducer applications.
It is desirable to create electromagnetic transducers utilizing magnets made of "hard" magnetic material which provide enhanced operating characteristics to the electromagnetic devices in which they are used and which take advantage of the amplification characteristics of "hard" ceramic magnetic materials.